I have a photo, which has to be revealed through mosaics/squares which are moving from the top left corner to the bottom right corner. While they move, it will reveal the picture. I hope its understandable what I mean. I don't need any 3D or camera effects. Just a simple photo reveal. At the beginning, the picture is white and you only see two squares on the top corner. Once they start moving more squares will appear and the original photo appears after them.

Is there a simple way to achieve this effect? Because I startet creating the squares bei hand and masking the white layer on top of the image. But this takes ages and I'm hoping, that there might be a faster way.

You would do multiple transitions immediately following one another. One of those transitions would be to a colored box composition and then that would immediately transition to the second footage comp.